The present invention relates to the processing of natural phosphates, and, more particularly, to a process for the production of hydrogen fluoride, phosphoric anhydride, calcium polyphosphates and nitric acid from natural phosphates.
Each of said products is widely used in the national economy.
Currently, the waste gases which are formed while processing natural phosphates to produce phosphoric fertilizer, phosphoric acids and defluorinated phosphates constitute a major source of commercial hydrogen fluoride (see: M. E. Pozin, Tekhnologiya mineral'nykh solei (Processing of Mineral Salts), Goskhimizdat, Leningrad, 1961, p. 725; V. V. Illarionov et al., Gidrotermicheskaya pererabotka fosfatov na udobreniya i kormovye sredstva (Hydrothermal Processing of Phosphates to Fertilizer and Fodder), "Khimiya", Moscow, 1965).
It is known in the art to employ two processes for the production of defluorinated phosphates, viz. fusing or sintering natural phosphates with a variety of additives, the rate of the defluorination process being markedly increased in the presence of water vapors.
Defluorination of natural phosphates and their mixtures with various additives is effected in rotary tube furnaces, electric furnaces and cyclones at temperatures of from 1,500.degree. to 1,800.degree. K. and reaction times of from fractions of an hour to 6 to 7 hours.
In view of the low intensity of the prior art processes for the defluorination of natural phosphates, defluorination is effected in large-capacity apparatus, resulting in the dilution of gaseous products, including fluorine, with huge quantities of the heat carrier gas. Thus, for instance, in hydrothermal defluorination of natural phosphates, the outgoing gas comprises a mixture of HF and SiF.sub.4. The concentration of this mixture in the outgoing gases, converted to pure fluorine, amounts to 0.5 to 8.0 g/cu.nm, depending on the kind of technology used in the process. The degree of defluorination is 96 to 98 percent.
The prior art processes for the defluorination of natural phosphates have the following disadvantages:
low rate of defluorination; PA1 low concentration of fluorine in the outgoing gases; PA1 the need for special preparation of the feedstock (granulation, introduction of additives). PA1 the process involves more than one step; PA1 the feedstock must undergo special preparation; PA1 the feedstock must be in a concentrated form; PA1 a premium reducing agent (coke) is used; and PA1 the process is highly capital intensive. PA1 the processes have a low rate and involve more than one step; PA1 the feedstock requires preparation; and PA1 commercial products, such as phosphorus, acids, etc., have to be employed as feedstock.
Phosphoric anhydride is currently produced by combusting in an air stream elemental phosphorus produced at the first step of carbon reduction of natural phosphates mixed with silica or other fluxes at a temperature of from 1,400.degree. to 1,700.degree. K. in a phosphorus electric furnace of a shaft type. The resultant gases containing gaseous phosphorus are purified on electric filters and condensed to form liquid phosphorus which is delivered to the combustion chamber where phosphorus is oxidized to form gaseous phosphorus anhydride. The latter is sent to spray towers for absorption to give phosphoric acid, or to special condensing towers where P.sub.2 O.sub.5 is condensed as a solid product.
Said process can only use highly concentrated natural phosphates as feedstock, for any reduction of the P.sub.2 O.sub.5 level in the feedstock is associated with a large increase in the power consumption and coke consumption rates, coke being the reducing agent. Additionally, the process requires the step of charge preparation, i.e. admixing the feedstock natural phosphate with silica and converting the mixture into a lumpy material by agglomeration, briquetting, etc. Agglomeration is needed to provide for a better contact of the reducing agent with the charge; hence, the phosphate fines are not utilized in the process and are returned to the first step of charge preparation.
The prior art process for the production of phosphoric anhydride has a number of disadvantages:
At present calcium polyphosphates are primarily produced by reacting natural phosphate with sulfuric acid or phosphoric acid to form monosubstituted orthophosphate.
The product monosubstituted orthophosphate is dehydrated by heating to 600.degree. K. to form polyphosphates, and then the resultant sintered mass is comminuted.
It is likewise known to produce calcium metaphosphate by reacting phosphoric anhydride with tri- or dicalcium orthophosphate at a temperature of 1,300.degree. to 1,500.degree. K.
According to the latter technique, lumpy or agglomerated natural phosphate is charged into a shaft furnace. Liquid phosphorus is sprayed and combusted in the combustion chamber disposed at the base of the shaft furnace. The phosphoric anhydride which is formed in the combustion of the liquid phosphorus together with the hot gases passes through a layer of natural phosphate and reacts therewith to form calcium metaphosphate which, being molten, flows downward and is periodically removed from the furnace. The melt is rapidly cooled, and the resultant product is comminuted and screened. Just as in the phosphorus production process, the feedstock has to be specially prepared by agglomeration in order to produde calcium metaphosphate.
The disadvantages of the foregoing processes for the production of polyphosphates are as follows:
At the present time, nitric acid is manufactured predominantly by the ammonia technique which comprises contact oxidation of ammonia. The process includes two steps: catalytic oxidatin of ammonia to nitrogen oxide on platinum catalysts, and oxidation of the nitrogen oxide to higher nitrogen oxides subsequently absorbed by water.
As can be seen, each of the products is manufactured according to to a special flow chart with the use of different commercial products (e.g. phosphorus, acids, coke), which adversely affects the cost of the end products.
It is economically advantageous, therefore, to have a process, wherein all said products could be continuously manufactured according to a single flow chart on the basis of cheap raw material and avoiding the use of other commercial products.